The present invention relates generally to digital color imaging, and more particularly to a method for reducing halo print defects occurring in printed/copied color images.
Electrophotographic marking is a well known and commonly used method of copying and printing documents. Electrophotographic marking is performed by exposing an image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to the image representation, the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor""s surface. Toner particles are then deposited onto the latent image so as to form a toner image. The toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
Electrophotographic marking can be used to produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to as a REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to an image representation of a first color separation, such as black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. The charge, expose, and develop process is repeated for a second color separation, such as yellow, then for a third color separation, such as magenta, and finally for a fourth color separation, such as cyan. The various color toner particles are placed in superimposed registration to form a composite toner image on the photoreceptor. The composite toner image is then transferred and fused onto a substrate to form a composite color image.
The REaD IOI process can be implemented in various ways. For example, the process can be implemented in a single-pass document processing system (e.g. color printer or color copier) wherein a final composite color image is produced in a single pass of the photoreceptor through the document processing system. A second implementation is in a multi-pass document processing system wherein the photoreceptive surface makes multiple passes through a single charging station and a single exposure station wherein only one color toner image is produced during each pass of the photoreceptor through the charging and exposure stations.
A problem known as a halo print defects can occur at the boundaries between different colored areas on a composite color image resulting from an IOI image forming process. FIGS. 1-4 illustrate the exemplary case of a first-colored, such as cyan, square area 10 having a second-colored, such as black, border area 12 surrounding the square. As shown in FIGS. 1 and 3A, the halo print defect can be seen as a space or gap 14 between the colored areas 10 and 12 of the composite toner image on the photoreceptor 16. In the resulting color image, the surface of the underlying substrate can be seen between the colored areas 10 and 12 because of the lack of toner deposited in the gap 14 on the photoreceptor 16.
However, as shown in FIGS. 2 and 3B, no such gaps exist between the colored areas 10 and 12 of the digital image representations that modulate an exposure device, such as a raster output scanner (ROS), to form the electrostatic latent images on the photoreceptor. Halo print defects are primarily caused by electrostatic interactions occurring at the boundary between two adjoining color areas of the composite toner image on the photoreceptor 16, and by color separation to separation registration errors.
Referring to FIG. 3C, trapping is a solution used in the printing industry to compensate for halo print defects primarily caused by color separation to separation registration errors. In trapping, the boundary 19 between two adjacent color areas 20, 22 of a printed document 24 is overlapped by one of the color areas, such as color area 20. Thus, making the color area 20 slightly larger eliminates the halo print defect in the resulting printed image.
A digital counterpart to trapping is disclosed in U.S. Pat. No. 5,581,667, issued to Steven J. Bloomberg and assigned to the same assignee of the present invention. In xerographic systems, however, trapping does not completely eliminate halo print defects because the toner gaps formed between adjacent color areas are due to a combination of registration errors and electrostatic interactions.
FIG. 4 maps the video values for each pixel of an 8-bit system (where a gray scale value of 255 is 100% on) along a given raster scan line through the color (CMYK) separations of the image objects 10 and 12. The value of the pixels on one side of the edge/boundary 18 in the black color separation are about 255, and the value of the pixels on the other side of the edge/boundary 18 in the cyan color separation are also about 255. It is suggested that the electrostatic interactions occurring along the boundary between the black toner image and the cyan toner image on the photoreceptive belt result in the occurrence of the halo print defects.
Accordingly, it has been considered desirable to develop a new and improved method for reducing halo print defects associated with printed and copied color images that meets the above-stated needs and overcomes the foregoing difficulties and others while providing better and more advantageous results.
The present invention provides a method for transitioning from a first-colored area to a second-colored area within a transition zone between the first-colored area and the second-colored area. In particular, the pixels values within the transition zone are ramped or otherwise sloped toward the edge/boundary to prevent the formation of halo print defects along the edge/boundary in the resulting multi-color output document.
Thus, in accordance with one aspect of the present invention, a method for reducing halo print defects in a color document generated by an image processing system is disclosed. The method includes a) detecting a boundary between a first image object and a second image object within a color separation of the color document, and b) modifying the video values of a plurality of pixels within the color separation so that the video values slope toward the boundary, the plurality of pixels being located within a transition zone adjacent the boundary.
In accordance with another aspect of the present invention, an image processing system is provided. The image processing system includes an image forming section for generating an composite toner image on a charge retentive surface, and an image processing section for processing pixel values associated with a plurality of color separations representing an original multiple-color image wherein the pixel values are transferred to the image forming section. The image processing section includes circuitry for detecting a boundary between a first image object and a second image object within a color separation, and circuitry for modifying the values of a plurality of pixels within the color separation so that the pixel values slope toward the boundary, the plurality of pixels being located within a transition zone adjacent the boundary.
One advantage of the present invention is the provision of a method for reducing the occurrence of print halo defects occurring in an output document.
Another advantage of the present invention is the provision of a method for eliminating the occurrence of print halo defects occurring in an output document.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiment.